1. Field of the Invention
The present invention relates semiconductor devices having solder balls as an external termination means and, more particularly, to a semiconductor device using a Ball Grid Array (BGA) having improved reliability solder joints and a method for manufacturing the same.
2. Description of the Related Art
A common mounting and electrical-connection mechanism used in integrated circuits (ICs) is a ball grid array (BGA), wherein small solder balls are placed and retained at each one of a multitude of connection pads of the IC for solder connection to a mounting substrate or planar surface having opposing metal connecting pads. When heat is applied, the solder balls liquefy and flow via a fluxing agent included in the solder compound over any exposed metal surfaces of the connection pads, thereby forming a reliable electrical connection with each mounting pad. After cooling, the hardened solder additionally provides a rigid mounting structure for mechanically retaining the IC to the substrate or planar surface.
Typically, the size of the mounting package for the IC can be reduced to the size of a semiconductor chip, thus creating a Chip Size Package or Chip Scale Package (CSP). In the CSP, unlike conventional periphery-leaded (i.e. wire-bonded) packages, an array of external terminals and the BGA solder balls are distributed over the surface of the IC to directly interconnect the package to a printed circuit board (PCB). After processing, the resulting solder structures are generally inelastic, and provide a solid mounting mechanism for the assembly.
Disadvantageously, since the material composition of the chip and the opposing epoxy-glass material of a conventional PCB can have widely mismatched Coefficient of Thermal Expansion (CTE), any thermal cycling effects, such as those normally associated with turn-on and turn-off of related circuitry, can produce differing expansion movement of the opposing planar surfaces. This movement produces lateral shearing stresses on the solder joint, which is absorbed by the solder balls, more specifically by the junction of the solder ball and the metal connecting pads. With repeated thermal cycling, metal fatigue at this junction can cause the solder structure to crack and fail, rendering an entire circuit board inoperable. In other words, when the chip heats up during use, both the chip and the board expand, and when the heat is removed, both the chip and the substrate shrink. The problem that arises is that the chip and the substrate expand and contract at different rates according the CTE, thereby stressing the interconnections or solder balls connecting them.
FIG. 1 illustrates a cross-sectional view of a conventional ball grid array (BGA) interface structure 10 having a multitude of solid solder balls 12. Structure 10 consists of upper planar element 14 having a first metal conductor pad 16 rigidly connected via solder ball 12 to a lower planar element 18 having a second metal conductor pad 20 so as to provide both electrical and mechanical connection between electronic circuits on each of the planar elements. Conductors are separated from lateral neighboring conductors by isolation spaces 22 appropriately located on each planar element. Each of planar elements 14 and 18 can have a different CTE. However, an excessive disparity between the CTEs can produce thermal cycling failure in the form of cracking of the rigid solder joints as previously discussed.
Occasionally, conventional out-gassing of vaporized flux is not completed due to process and/or solder compound irregularities, such as insufficient wetting of the solder compound, and results in small voids being located at the conductor-solder interface. Such voids create an area of fatigue weakness in the resulting joining structure.
FIG. 2 illustrates a cross-sectional area of a conventional BGA structure, wherein small voids 24 due to insufficient conventional wetting are shown at the junction of conductor pad 16 and solder ball 12. Such voids 24 are formed when each of the molten solder balls 12 wets on the conductor pad 16, and the flux in the solder paste compound flows outward from the center of the conductor pad 16. As the temperature of the solder compound rises further, the flux is vaporized and a major portion of the flux vapor is dissipated into the atmosphere. However, minor portion of this vapor remains trapped in solder ball 12 as it is cooled and forms the small voids 24 inside the solder ball 12.
The configurations shown in FIGS. 1 and 2 are susceptible to cracking at each joint connecting the rigid conductor pad 16 and the rigid solder ball 12 under application of opposing lateral forces on the planar elements 14 and 18. Such a failure of both the electrical connection and the mechanical mounting mechanism has heretofore precluded the use of epoxy-glass as a reliable substrate material for BGA applications, in favor of a more expensive ceramic material which has a CTE closer to that of the chip. A detailed joint interface of solder balls used in a conventional BGA type semiconductor device mounting construction is disclosed, for example, in U.S. Pat. No. 6,122,177 and PAJ No. 1998-209591. Detailed manufacturing assembly technical reports from International Business Machines Corporation, entitled xe2x80x9cDoubled-Sided 4 Mb SRAM Coupled Cap PBGA Card Assembly Guide,xe2x80x9d and from Texas Instrument Corporation, entitled xe2x80x9cMicroStar BGA Packaging Reference Guide,xe2x80x9d would also be beneficial to the reader.
The prior art addresses the presence of these small voids in the solder balls and a resulting joint embrittlement as significant problems. A small void in this context is defined as a gaseous volumetric displacement within the interior of a solder ball due to thermal expansion (i.e. boiling) of low-temperature solder flux solvents, since such gas material will remain trapped within a cooled solder structure. Conventional solder processing typically incorporates a warm-up period to allow time for de-gassing of such solvents, thereby minimizing such voids to yield a recommended finished total gaseous volume of less than 0.1% of the total solder structure volume. Disadvantageously, such heating can prematurely dry the solder paste included in the solder ball, leading to degraded electrical connections.
Thus, conventional BGA structures are not sufficient to prevent solder cracking or the breakage of solder ball interconnection, especially when used with a chip and an epoxy-glass substrate. Therefore, what is needed is a newly designed CSP with improved interconnection reliability, especially between the chip and the PCB, and a method of manufacturing of the same.
In a preferred embodiment of the present invention, improved reliability of solder ball connections in a ball grid array (BGA) semiconductor device can be attained by forming a large cavity in each finished solder ball structure or eliminating small voids in the board land structure of the annular metal patterns. Such cavities can be controllably formed via a seeding catalyst in the form of an annular ring in contact with a solder ball having a volatile fluxing agent. During melting of the solder ball, the vaporized fluxing agent accumulates around the non-conductive hole in the annular ring and effectively xe2x80x9cinflatesxe2x80x9d the solder ball, such that, when cooled, a hollow solder structure having flexible thin walls results that can absorb subsequent lateral movement of the opposing planar elements without degrading the solder joint. This is particularly useful in applications where thermal stresses generated during the thermal cycling can be absorbed or dissipated efficiently without breakage or degradation of the joints (physical connection) between the hollow conductive solder balls and the underlying structure.
Accordingly, a solder structure according to preferred embodiments of the present invention provides added resiliency to the overall solder bond by inducing the creation of a large gaseous cavity within the resulting solder structure. The presence of such a cavity produces a thin walled xe2x80x9cbarrelxe2x80x9d shape solder structure that has sufficient resiliency and flexibility to xe2x80x9cbendxe2x80x9d under laterally applied stresses, thereby removing stress vectors that would occur at each joint between a conductor pad 16 and a conventional solder ball 12.
It is a feature of a preferred embodiment of the present invention to provide a solder structure for electrically and mechanically connecting a first metal contact on a first planar surface to a second metal contact on a second planar surface, the solder structure comprising a solder element having a curved exterior surface enclosing a first volume and an interior cavity having a displacement constituting a second volume. The solder structure consists of hollow cylindrical solder structure that can be circular (produced by a circular contact pad on one planar surface and an annular contact pad on the opposing planar surface) or xe2x80x9cbarrel-shapedxe2x80x9d (produced by an annular contact pad on both opposing planar surfaces), further comprising a cylindrical exterior wall connected to conductive pads on each of the two planar surfaces being joined, and an interior cavity having a volumetric displacement that comprises between 1 and 90% of the total volumetric displacement of the solder structure. A solder compound can be used to attain the above resilient solder structure that can be comprised of a mixture of one or more from the group consisting of solder, silver, tin, and a solder fluxing, comprising one or more from the group consisting of rosin, resin, activator, thixotropic agent, and a high temperature boiling solvent.
It is another feature of a preferred embodiment of the present invention that a plurality of geometric configurations of the conductive pads in contact with the solder balls can cause cavities to form at the center of such geometries during heating of the solder ball to at least the melting point and subsequent cooling. One such geometry is an annular ring having a center hole sufficiently large to overcome the surface tension of a liquid drop of solder resting at wetting equilibrium on a solid surface. One such geometry features the diameter of the hole in annular metal land pattern as being less than 90% of the outer diameter of the annular metal land pattern. Another geometry features a diameter of the solder ball as being greater than the outer diameter of the annular metal land pattern. Another geometry features a mass of the solder material as being less than a mass of the fluxing material. Still another geometry features a weighted proportion of the solder material as being less than a weighted proportion of the fluxing material.
It is another feature of a preferred embodiment of the present invention to provide a method for creating such a resilient solder structure, such method comprising the steps of: 1) etching the first metal contact, further comprising an annular land pattern; 2) etching the second metal contact, further comprising a circular land pattern; 3) placing a generally spherical soldering means in contact with the annular land pattern; 4) positioning the second planar surface parallel to the first planar surface with the second metal contact contacting the spherical solder means; 5) applying a heating means to change the state of the soldering means to a molten state; 6) maintaining temperature for a predetermined amount of time in order for the molten solder to reflow across the surface of the first and second metal contacts; and 7) reducing the heat to allow the soldering means to return to a solid state. The method can additionally include a step of etching a hole in the second metal contact to create an annular ring. This step in combination with the opposing annular ring can be used to create the barrel-shaped solder structure.
These and other features of the present invention will be readily apparent to those of ordinary skill in the art upon review of the detailed description that follows.